1. Field of the Invention
The present invention relates to a position calculating apparatus that uses global positioning system (GPS) satellites and a self-contained sensor to calculate a position of a moving body and, more particularly, to a method of determining a movement angle of a moving body.
2. Description of the Related Art
Navigation apparatuses widely use GPS positioning that uses GPS satellites and self-contained navigation that uses, for example, gyro sensors or vehicle-speed sensors mounted in vehicles in order to calculate positions of the vehicles. The GPS positioning can detect absolute positions or orientations but the detection accuracy can be decreased depending on the reception environments. In contrast, the self-contained navigation does not depend on the reception environments but has the disadvantage of accumulated error. The navigation apparatuses use both the GPS positioning and the self-contained navigation in complementary fashion to calculate positions of the vehicles.
Japanese Unexamined Patent Application Publication No. 10-307036 relates to a hybrid navigator system that uses data from a self-contained sensor and a GPS device to yield navigation data about a moving body. The navigator system calculates the navigation data on the basis of data from the self-contained sensor and data received by the GPS device at the same positioning time. Since the GPS device is not provided with a calculation unit that calculates positional data including latitude, longitude, and orientation from the received data, unlike GPS devices in the related art, it is not necessary to perform a calculation for accommodating a difference in time between the data received from the self-contained sensor and the data received by the GPS device in the navigator system.
Japanese Unexamined Patent Application Publication No. 2003-279362 relates to a vehicle position correction device. The vehicle position correction device compares a traveling distance during a certain time period, yielded by self-contained navigation, with a distance between GPS positioning points during a time period corresponding to the certain time period, yielded by GPS positioning. If the difference between the traveling distance yielded by the self-contained navigation and the distance between the GPS positioning points is large, the vehicle position correction device determines that the GPS positioning data has a lower reliability. If the difference is small, the vehicle position correction device determines that the GPS positioning data has a higher reliability. The radius of an error circle indicating a range permitted for the GPS positioning error is decreased if the reliability is high while the radius is increased if the reliability is low.
Japanese Unexamined Patent Application Publication No. 2004-150852 relates to a satellite signal receiver. The satellite signal receiver adds a velocity vector Vn−1 yielded from the previous speed and speed orientation to the previously calculated position Xn−1 to calculate an estimated position Yn, yields the current calculated position Zn at a positioning unit, and calculates the reliabilities of the estimated position Yn and the calculated position Zn. Then, the satellite signal receiver yields a weight Tn to be assigned to the estimated position Yn from the reliabilities of the estimated position Yn and the calculated position Zn to calculate a corrected position Xn. The satellite signal receiver calculates an optimal corrected position from the reliability of the calculated position and the reliability of the estimated position each time the positioning is performed so that the error in speed is not accumulated, in order to improve the positioning accuracy.
As disclosed in Japanese Unexamined Patent Application Publication No. 2004-150852, if the error in the orientation defining the velocity vector is large when the satellite signal receiver calculates the reliability of the estimated position, yielded by adding the velocity vector to the previous calculated position, and the reliability of the current calculated position at the positioning unit to determine the corrected position on the basis of the reliabilities, the error in the estimated position becomes large, thus reducing the accuracy of the corrected position. Accordingly, in order to increase the accuracy of the corrected position, it is necessary to improve the accuracy of the orientation or movement angle defining the velocity vector.
The speed orientation defining the velocity vector can be obtained from absolute orientation data yielded by GPS positioning or relative orientation data yielded by self-contained navigation. The absolute orientation data is yielded by Doppler shift. The absolute orientation data that is almost correct can be obtained in a case where the positioning is performed in a good positioning environment, as in the open air. However, the error in positioning becomes large in a case where the vehicle is stopped or in a case affected by, for example, multipath, as in a shielded environment. In contrast, the relative orientation data does not depend on the reception environment, unlike the GPS positioning, and offers a variation in orientation that is almost correct. However, the error is accumulated in the case of continuous use for a long time or in a ease where the orientation varies significantly, as in a winding route.
In order to overcome the disadvantages of both the absolute orientation data and the relative orientation data, it is desired that the orientation data having a higher reliability, of the absolute orientation data and the relative orientation data, be adopted for the velocity vector each time the positioning is performed.